Apparatus and method for TFT fingerprint sensor

ABSTRACT

A low cost, two-dimensional, fingerprint sensor includes a pixel array, each pixel including a switch and a pixel electrode for forming a capacitance with a fingertip. One or more active transmission electrodes are spaced from a selected row of the pixel array, and transmit a carrier signal into the finger without direct coupling into the selected pixels. Signals sensed by the pixel array are coupled to an independent integrated circuit, and connections between the IC and the pixel array are reduced by demultiplexing row select lines, and by multiplexing sensed column data. Differential sensing may be used to improve common mode noise rejection. The fingerprint sensor may be conveniently incorporated within a conventional touchpad LCD panel, and can mimic the performance of lower density touchpad pixels.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of pending U.S. patent application Ser.No. 14/244,534, filed on Apr. 3, 2014 and entitled “APPARATUS AND METHODFOR TFT FINGERPRINT SENSOR”, which claims the benefit of U.S.Provisional Application No. 61/820,477, filed on May 7, 2013, theentirety of which are incorporated by reference herein.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates generally to electronic fingerprintsensors, and more particularly to a fingerprint sensor using a thin-filmtransistor (“TFT”) array to capacitively sense a user's fingerprint.

2. Description of the Related Art

Conventional fingerprint sensors currently being sourced to thecommercial market use different methods for sensing a user'sfingerprint. One type of fingerprint sensor includes a CMOS silicon chiphaving circuitry providing an arrayed multitude of “pixels”. The CMOSsilicon chip is then coated with a protective coating that may be formedfrom a simple chemical coating, a flex substrate, or other thinmaterials. This type of fingerprint sensor requires the silicon chip tobe at least as large as the pixel array.

A second type of commercially available fingerprint sensor includesmetal lines formed upon a substrate to form a pixel array, while aremotely-located silicon chip, of smaller dimensions than the pixelarray, is electrically coupled thereto. This second type of fingerprintsensor can be implemented in several different packaging configurations,such as ball grid array (BGA), wafer level fan-out (WLFO), or a filmsubstrate conformed around, or on top of, a plastic hump/stiffener.

Fingerprints are characterized by patterns of ridges and valleys thatare present on the skin of a user's finger. Most of the currentcommercial fingerprint sensors are capacitive touch sensors, meaningthat the circuitry used to derive a fingerprint image must be capable ofdifferentiating small changes in a received signal that result from thecapacitance induced by a finger “ridge” or “valley” positioned over theplate of a capacitive sensing element. These capacitive sensing elementsare typically laid out in an array of X rows by Y columns, commonlyreferred to as a “pixel array”. The intersection of each row with eachcolumn is referred to as a “pixel”. These pixel arrays can be created byCMOS devices formed in a semiconductor integrated circuit chip itself,as in the first type of fingerprint sensor described above.Alternatively, the pixel array can be formed by metal lines formed uponon a non-semiconductor substrate material, as in the second type offingerprint sensor described above.

The first type of fingerprint sensor described above results in a muchhigher-cost product, since the CMOS silicon chip in which the pixelarray is formed must be at least the size of the fingerprint imagerequired. In the case of a touch sensor, or 2D sensor, this can requirea relatively large area of silicon, measuring three-quarters of an inchsquare or larger, making it relatively costly.

On the other hand, the second type of fingerprint sensor describedabove, wherein metal traces are formed upon a non-semiconductorsubstrate, often develop an inferior signal due to the limitations onthe widths of the lines used to transmit and receive the signal fromwhich the fingerprint image is derived. The smaller sizes of thetransmitter and receivers, especially the transmitter, can also severelylimit the thickness of material above the sensor.

A third type of fingerprint sensor that has been proposed uses a liquidcrystal display (LCD), which is ordinarily used to display, rather thansense, information. In this third type of sensor, the LCD display itselfis used to image and capture the fingerprint, providing a single devicethat is both a display and fingerprint sensor. This method proposes theuse of the “column drivers” of the display to not only outputinformation but to have an input mode that can sense a capacitancechange on the pixels in the display. This method is extremely limited insignal strength because the column lines must be used as bothtransmission (Tx) lines and receiver (Rx) lines. For example, it hasbeen proposed to use a “pre-charge” state on each pixel before theuser's finger is applied, and then to detect the voltage change on eachsuch pixel in the presence of the user's finger, thereby monitoring thecapacitance provided by the ridge or valley of the finger over thatpixel. The use of the column line as both Tx (precharge) and Rx(receive, or read) severely limits the signal to noise capabilities thatthis method can produce. In addition, this method is costly, as all ofthe column drivers must be designed to serve as both an output device(for normal display usage) and a highly sensitive input device (forfingerprint sensor usage).

In order for the above-described sensors to properly distinguish the“ridge” versus “valley” signal delta, the finger must be located asclose as possible to the receiver plate(s) of the capacitor.Accordingly, suppliers of known fingerprint sensors strive to minimizethe thickness of the receiver plate that overlies the capacitive plateof each pixel. However, as the receiver plate thickness is reduced, suchfingerprint sensors are more easily damaged physically or mechanicallybecause of the close proximity of the sensor surface to underlyingelectrical circuitry, thus reducing the durability and/or reliability ofthe sensor. For example, conventional BGA-style fingerprint sensors, aswell as newer, more advanced “flexible” fingerprint sensors, whichenable a user to swipe a finger across a polyimide surface withoutdirectly contacting the sensor circuitry, are both susceptible to thistype of damage.

As explained above, current fingerprint sensors require that the user'sfingertip be in close proximity to the fingerprint sensor circuitry inorder to sufficiently distinguish the ridges and valleys of thefingertip. Accordingly, for the second type of fingerprint sensordescribed above, the thickness and material type used to protect thefingerprint sensor is severely limited. The protective coatingscurrently used to cover fingerprint sensors must be non-conductive, lessthan approximately 200 um thickness, and fit the aesthetic requirementsof the customer. For example, a simple drop of a pen striking theexposed portion of the fingerprint sensor can damage the thin polyimidesurface of the flexible fingerprint sensor, thus creating aestheticdefects and potentially damaging the sensor circuitry located just belowthe surface. The ability to place thicker materials over the sensor toadd to the reliability of the fingerprint sensor is highly desirable.However, thicker protective coatings/surfaces introduce at least two newchallenges: 1)—the signal strength from the signal transmitter, to thefinger, then back to the receiver array, is greatly diminished asthickness of the cover increases, typically by the square of theincreased distance (i.e., doubling the cover thickness reduces signalstrength by a factor of four); and 2)—depending on how the transmittersignal is generated, the transmitter signal can be significantlyde-focused as it travels from the transmitter to the receiver.

Among the fingerprint sensors that are currently available is a “GlassCap Sensor” supplied by Silicon Display of South Korea under Model No.GCS-2. This device provides a poly silicon thin film transistor (TFT)capacitive pixel array of 256 rows by 360 columns, corresponding to92,160 sensor cells. The pixel density corresponds to 508 dpi, and isprovided within a sensing area measuring 12.8 mm by 18 mm. The pitchbetween successive pixels in the array is 350 micrometers. A gate/rowshift register is formed on the integrated circuit and is used to selectthe active row of pixels to be sensed. Likewise, a column shift registeris formed on the integrated circuit for selecting the columns to besensed within the selected row. Four analog output sensing signals areprovided at any given point in time. A multiplexer is also formed on theintegrated circuit, and is used to select which column output sensingsignals are selected at any given point in time. Applicants believe thatthe above-described Glass Cap Sensor is essentially a passive devicethat does not include any signal generating electrodes for radiating ahigh frequency signal proximate to the pixel array in order to detectthe effective capacitance formed between each of the pixels of the arrayand the user's fingertip.

U.S. Pat. No. 6,055,324 to Fujieda discloses a fingerprint imagingdevice including a two-dimensional array of thin film transistors (TFTs)formed within a substrate, a dielectric layer formed above suchsubstrate, and signal sensing electrodes formed on the dielectric layer.The signal sensing electrodes are connected to the source terminals ofthe thin film transistors.

The gate electrodes of TFTs lying within the same row of the array areconnected to a common gate electrode lead. The gate electrode leads areconnected to output terminals of a shift register used to select whichof the rows of the array is active. The drain electrodes of TFTs lyingin the same column are connected to a common drain electrode lead. Thedrain electrode leads are connected to input terminals of a signaldetecting circuit. A signal generating electrode is provided in the formof mesh or comb for surrounding the pixels of the two-dimensional arrayand for radiating a high frequency signal toward a finger overlying thearray. The signal sensing electrodes of the array form electrostaticcapacitances between the signal sensing electrodes and the user'sfinger. The signal received by each of the signal sensing electrodes isdetected, row by row, to provide an image of a fingerprint. However, inFujieda, the signal generating electrode is so highly enmeshed with eachof signal sensing electrodes of the array that significant components ofthe radiated high frequency signal are directly capacitively coupled tothe signal sensing electrodes without first passing through the user'sfinger. As a result, the difference in signal strength between a firstsignal sensing electrode lying below a ridge of the user's fingertip,and a second signal sensing electrode lying below a valley of the user'sfingertip, is not nearly as pronounced as it should be. Moreover, as thethickness of the protective layer, separating the user's finger from theunderlying signal sensing electrodes, is increased, the directcapacitive coupling of the radiated high frequency signal from thesignal generating electrode to the array of signal sensing electrodeslargely overwhelms any secondary coupling of the radiated high frequencysignal through the user's finger.

As evidenced by the purchase of Authentec by Apple, the fingerprintsensor is a biometric security system with great potential in the cellphone, notebook, and laptop arena. Thus, the ability to embed afingerprint sensor in an LCD panel, or to create a fingerprint sensor ina component which is common to many of these media, such as a button, ishighly desirable.

Accordingly, it is an object of the present invention to provide afingerprint sensor for imaging a person's fingerprint without requiringthe use of an integrated circuit semiconductor chip of the samedimensions as the pixel array used to capture the image of thefingerprint.

Another object of the present invention is to provide such a fingerprintsensor which more readily distinguishes between the ridges and valleysof a fingertip applied to a cover plate overlying the pixel array usedto image the fingerprint.

Still another object of the present invention is to provide such afingerprint sensor wherein the cover layer, or coating, overlying thepixel array can be made of sufficient thickness to adequately protectthe pixel array while still permitting the pixel array to readilydistinguish between the ridges and valleys of an applied fingertip.

A further object of the present invention is to provide such afingerprint sensor which can be manufactured at relatively low cost.

Yet a further object of the present invention is to provide such afingerprint sensor which more effectively transmits a carrier electricalsignal into the person's fingertip without simultaneously directlycoupling such carrier signal into the pixel array.

A still further object of the present invention is to provide such afingerprint sensor which reduces the number of electrical lines betweenthe pixel array and an associated integrated circuit used to process thefingerprint image captured by the pixel array.

Yet another object of the present invention is to provide such afingerprint sensor wherein the signal components monitored by each pixelwithin the pixel array can be sensed differentially to reject commonmode noise signals.

Still another object of the present invention is to provide such afingerprint sensor wherein the pixel array may be incorporated as aportion of a conventional touch-sensitive pad.

Another object of the present invention is to provide a fingerprintsensor which readily transmits a signal into the user's finger that canbe sensed by the pixel array, but wherein the transmitted signal is notsignificantly directly coupled to the pixel array through thefingerprint sensor itself.

A still further object of the present invention is to provide afingerprint sensor which can be easily combined with a conventionaltouchpad to provide a single device which can both image a user'sfingerprint and detect that the user is touching a particular locationof the touchpad, within the same sensing layers.

These and other objects of the present invention will become moreapparent to those skilled in the art as the description of the presentinvention proceeds.

SUMMARY OF THE INVENTION

Briefly described, and in accordance with a preferred embodimentthereof, the present invention relates to a fingerprint sensor thatincludes a first substrate having a two-dimensional array of pixelsarranged in R rows and N columns. The first substrate may either berigid or relatively flexible. Each pixel includes a switching device,preferably a TFT, and a capacitive plate proximate the upper surface ofthe first substrate. A series of R row addressing electrodes areprovided, each row addressing electrode being coupled to the switchingdevices of the pixels in a corresponding row of the pixel array toselectively enable the switching devices in the corresponding pixel row.A series of N data electrodes are also provided, each data electrodebeing coupled to the switching devices of the pixels in a correspondingpixel column for sensing the signal provided by the capacitive plate ofthe pixel located at the intersection of the selected pixel row and thecorresponding column of the pixel array.

One or more transmitter electrodes are formed proximate to the uppersurface of the first substrate for transmitting a varying amplitudeelectrical signal. In one embodiment, the

transmitter electrode is laterally spaced apart from the pixel array,and preferably extends substantially entirely around the perimeter ofthe pixel array. A cover layer overlies the upper surface of the firstsubstrate for receiving a fingertip of a user; if desired, the coverlayer may be integrally formed with the first substrate. The varyingamplitude electrical signal transmitted by the transmitter electrode iscoupled into a finger of a user who places his or her fingertip over thecover layer, and wherein the electrical signal coupled into the user'sfinger is further coupled through the capacitive plates in the pixelarray to a greater or lesser extent, depending upon whether a ridge orvalley of the user's fingertip overlies a particular pixel in the pixelarray.

According to another embodiment of the present invention, multipletransmitter electrodes are located within the borders of the pixelarray, preferably interspersed between successive pixel rows.Selectively-enabled transmission electrodes transmit a carrier signalfor transmission into the user's finger. Transmission electrodesneighboring upon a selected row of the pixel array are disabled, whiletransmission electrodes more distant from the selected row of the pixelarray are enabled, and transmit the desired carrier signal. This allowsfor effective transmission of the carrier signal into the user'sfingertip, without any significant direct coupling of the carrier signalto the pixels within the currently selected row. Each time a new row isselected, the enabling and disabling of the transmission electrodes isupdated to ensure that the transmission electrodes neighboring theselected row are disabled, and that the more distant transmissionelectrodes are actively transmitting the carrier signal.

In accordance with an alternate embodiment of the present invention, thefingerprint sensor is incorporated within a touchpad, wherein thetouchpad includes a substrate. An array of sensor pixels are formed inthe substrate and arranged along intersecting rows and columns forsensing the presence and location of a finger, stylus, or other“pointer” applied proximate to the upper surface of the substrate.Adjacent sensor pixels are spaced apart from each other by a firstpredetermined distance, corresponding to a first pitch. Each sensorpixel provides a signal indicating whether a pointer is being appliedproximate to such sensor pixel.

The touchpad includes a series of row address lines supported by thesubstrate. Each row address line is associated with a row of sensorpixels in the array to selectively enable and address the sensor pixelsin such row. The touchpad also includes a series of column sensing linessupported by the substrate. Each column sensing line is associated witha column of

sensor pixels for sensing a signal provided by a sensor pixel in the rowof sensor pixels selected by an enabled row address line.

A fingerprint sensor area is formed upon the touchpad substrate. Thefingerprint sensor area includes a series of finer-pitch pixels arrangedalong intersecting rows and columns to form an array of finer-pitchpixels. Each finer-pitch pixel includes a switching device and acapacitive plate. Each finer-pitch pixel is spaced apart from adjacentfiner-pitch pixels by a second predetermined distance, the secondpredetermined distance being less than one-third the first predetermineddistance that separates the sensor pixels of the touchpad.

In order to detect the image of the user's fingerprint within thefingerprint sensor area, a series of finer-pitch row address lines areprovided. Each finer-pitch row address line is associated with a row offiner-pitch pixels in the finer-pitch pixel array, and each finer-pitchrow address line is selectively enabled to address the finer-pitchpixels associated therewith.

Likewise, a series of finer-pitch column sensing lines are provided,each finer-pitch column sensing line being associated with a column offiner-pitch pixels. The finer-pitch column sensing lines serve to sensea signal provided by the capacitive plate of a finer-pitch pixel in anenabled row of finer-pitch pixels.

The fingerprint sensor area of the touchpad is bordered by touchpad rowaddress lines and touchpad column sensing lines. Ideally, at least oneof the touchpad row address lines and/or touchpad column sensing linesthat borders the fingerprint sensor area also serves as a transmitterelectrode for transmitting a varying amplitude electrical signal whenthe fingerprint sensor area is being used to sense a user's fingerprint.The varying amplitude electrical signal transmitted by the transmitterelectrode is coupled into a finger of a user who places his or herfingertip over the fingerprint sensor area. The electrical signalcoupled into the user's finger is further coupled through the capacitiveplates in the finer-pitch pixel array to a greater or lesser extent,depending upon whether a ridge or valley of the user's fingertipoverlies a particular pixel in the finer-pitch pixel array.

In yet another embodiment of the present invention, the aforementionedfingerprint sensor area can mimic the operation of the touchpad sensorsduring those times when a fingerprint image is not required. As before,the touchpad includes an array of sensor pixels

arranged along intersecting rows and columns, and spaced apart from eachother by a first predetermined distance. The touch pad also includes aseries of row address lines, each row address line being associated witha row of sensor pixels in the array. Each row address line isselectively enabled to address the sensor pixels associated with eachsuch row. The touchpad further includes a series of column sensinglines, each column sensing line being associated with a column of sensorpixels to sense the signal provided by a sensor pixel in the addressedrow of sensor pixels.

As before, the touchpad includes a fingerprint sensor area having anarray of finer-pitch pixels arranged along intersecting rows andcolumns. Each such finer-pitch pixel includes a switching device and acapacitive plate. Each finer-pitch pixel is spaced apart from adjacentfiner-pitch pixels by a second predetermined distance, the secondpredetermined distance being less than one-third the first predetermineddistance that separates the sensor pixels of the touchpad.

Just as in the case of the previously-described embodiment, a series offiner-pitch row address lines are provided. Each finer-pitch row addressline is associated with a row of finer-pitch pixels in the finer-pitchpixel array, and each finer-pitch row address line is selectivelyenabled to address the finer-pitch pixels associated therewith.Likewise, a series of finer-pitch column sensing lines are provided,each finer-pitch column sensing line being associated with a column offiner-pitch pixels. The finer-pitch column sensing lines serve to sensea signal provided by the capacitive plate of a finer-pitch pixel in anenabled row of finer-pitch pixels.

A control circuit, responsive to a mode signal, is also provided fordetermining whether the finer-pitch pixels are to function asfingerprint sensing pixels or conventional sensor pixels of thetouchpad. The control circuit individually enables the finer-pitchpixels in each finer-pitch row. When functioning as a fingerprintsensor, the signal provided by each finer-pitch pixel is individuallysensed. On the other hand, when the mode signal indicates that thefiner-pitch pixels are to function as conventional sensor pixels of thetouchpad, the control circuit simultaneously enables the finer-pitchpixels in a multitude of adjacent finer-pitch rows, and collectivelysenses the signals provided by the finer-pitch pixels in thesimultaneously enabled finer-pitch rows to simulate, or mimic, theoperation of a conventional sensor pixel of the touchpad.

Another aspect of the present invention relates to a fingerprint sensorincluding a demultiplexer for reducing the number of conductors thatmust extend between the pixel array and a related integrated circuit. Inthis regard, the fingerprint sensor includes a first substrate, whichmay be rigid or relatively flexible, having formed therein atwo-dimensional array of pixels arranged in R rows and N columns. Eachof such pixels includes a switching device (e.g., a TFT), which ispreferably a thin film transistor, and a capacitive plate. A series of Rrow addressing electrodes extend across the pixel array, each rowaddressing electrode being coupled to switching devices in acorresponding row of the pixel array for selectively enabling theswitching devices in such row. Likewise, a series of N data electrodesare provided, each data electrode being coupled to the switching devicesof the pixels in a corresponding column of the pixel array. Each dataelectrode senses a signal provided by the capacitive plate of the pixellocated at an intersection of a selected pixel row and the correspondingcolumn of the pixel array. A cover layer overlies the first substratefor receiving a fingertip of a user; this cover layer may be integrallyformed with the first substrate, if desired.

The fingerprint sensor of this embodiment of the invention also includesa second substrate different from the first substrate, and includingsemiconductive material to form an integrated circuit. The integratedcircuit generates a set of S row addressing signals to address one ofthe R row addressing electrodes. A demultiplexer is coupled between theintegrated circuit and the pixel array. The demultiplexer includes atleast S input terminals for receiving the first set of S row addressingsignals provided by the integrated circuit, and includes at least Routput terminals. Each of the R output terminals of the demultiplexer iscoupled to one of the R row addressing electrodes. The demultiplexerselects one of the R row addressing electrodes based upon the S rowaddressing signals received thereby. The integrated circuit is alsoselectively coupled to the N data electrodes to receive the signalsprovided by the capacitive plates in the pixel array. Preferably, thedemultiplexer is configured from a series of switching devices similarto those provided within the pixel array. The switching devices used toform the demultiplexer may be thin-film-transistors formed upon thefirst substrate.

Apart from incorporating a row-address demultiplexer, the fingerprintsensor described above may also include a multiplexer coupled betweenthe integrated circuit and the column electrodes of the pixel array. Inthis regard, the integrated circuit generates a set of M columnselection signals to address one of the plurality of N data electrodes.The multiplexer includes a first set of N input terminals each beingcoupled to a respective one of the N data electrodes to receive thesignals provided by the capacitive plates in the pixel array. Themultiplexer also includes a second set of M input terminals forreceiving the M column selection signals provided by the integratedcircuit. Based upon the status of the M column selection signals, themultiplexer selects at least one of the N data electrodes to detect thesignal provided by the capacitive plate located at the intersection ofthe selected row and column of the pixel array. Preferably, themultiplexer also includes an output terminal coupled to the integratedcircuit for providing a selected data signal thereto. As in the case ofthe row-address demultiplexer described above, the column electrodemultiplexer may be configured from switching devices (e.g.,thin-film-transistors) formed upon the first substrate.

Ideally, the fingerprint sensor having a row-address demultiplexer, asdescribed above, also includes at least one transmitter electrodesupported by the first substrate for transmitting a signal ofpredetermined frequency and amplitude proximate to the pixel array. Thetransmitted signal passes through the cover layer overlying the pixelarray and into the fingertip of the user for being coupled to thecapacitive plates of the pixel array through the ridges and valleys ofthe user's fingertip. As already noted above, the transmitter electrodemay take the form of a ring encircling the periphery of the pixel array.

Yet another aspect of the present invention regards a fingerprint sensorwherein signals that are capacitively coupled from the user's fingerinto the pixel array are sensed in a differential manner to help blocknoise signals. As in the preferred embodiments described above, thefingerprint sensor includes a first substrate having a two-dimensionalarray of pixels arranged in R rows and N columns. Each such pixelincludes a switching device (e.g., a TFT) and a capacitive plate. Onceagain, a series of R row addressing electrodes are provided, each rowaddressing electrode being coupled to switching devices within a row ofthe pixel array for selectively enabling the switching devices in thecorresponding pixel row. Similarly, N data electrodes are provided, eachdata electrode being coupled to switching devices in a correspondingcolumn of the pixel array for sensing a signal provided by thecapacitive plate of the pixel located at an intersection of a selectedpixel row and the corresponding column of the pixel array. A cover layeroverlies the first substrate for receiving a fingertip of a user.

In one instance, a common electrode is provided. The common electrodeextends at least partially through the pixel array formed on the firstsubstrate. A series of differential amplifiers are provided fordifferentially sensing signals being passed by the capacitive plates ofthe pixel array. Each differential amplifier has a first input coupledto one of the data electrodes, and a second input coupled to the commonelectrode. In addition, each differential amplifier has an outputterminal for providing an output signal representative of the differencebetween a signal provided by a data electrode and a signal provided bythe common electrode.

In a second instance, a separate common electrode is omitted, and one ofthe data electrodes serves double-duty as a reference electrode. Eachdifferential amplifier has a first input coupled to one of the dataelectrodes, and a second input coupled to the reference electrode. Eachdifferential amplifier has an output terminal for providing an outputsignal representative of the difference between the data signal providedby its corresponding data electrode and a signal provided by thereference electrode.

Whether the differential-type fingerprint sensor uses a common electrodeor a reference electrode, it preferably includes a second substrate ofsemiconductive material. This second substrate is different from thefirst substrate, and an integrated circuit is preferably formed withinthe second substrate to provide control logic.

As noted above, the preferred form of fingerprint sensor includes atransmitter electrode with one or more metal traces for transmission ofa high frequency signal. The transmitter may be a single trace orseveral traces in a variety of patterns, including, but not beinglimited to, a ring; however, the traces being used to transmit suchsignal, at any given point in time, are preferably laterally spaced fromcapacitive plates within the pixel array that are being sensed at thesame point in time, to avoid direct signal coupling from the transmitterelectrode to the capacitive plate being sensed. The transmitterelectrode is used to radiate a signal which is sent into the body of thefinger. Thus, the location of the transmitter electrode is preferablyclose enough to the finger to allow for the signal to penetrate thefinger, but far enough away from the active capacitive plates in thepixel array to prevent unwanted receptions that do not travel throughthe finger. While such unwanted receptions can, at least in theory, becalibrated-out by first transmitting the high frequency signal with nofinger present, and recording the baseline reception energy, imaging ofthe fingerprint is simpler, and more accurate, if such unwantedreceptions are avoided in the first instance. Because the signal isbroadcast into the entire finger, and the resulting transmission energyis sent through the entire finger to the receiver array, the signalstays relatively focused as it leaves the ridges and valley of thefinger and travels through the relatively thick cover plate material andonto the capacitive plates of the pixel array.

The transmitter electrode may be formed using any metal layer alreadyavailable in the fingerprint sensor area, or it may be an added layer orcomponent. The transmitter electrode may be part of a liquid crystaldisplay (LCD), or external to it. The amplitude and frequency of thesignal being broadcast by the transmitter electrode can be varied tobest suit a particular environment. The transmitter drive circuitry canbe located within the IC or external to the IC.

As noted above, a fingerprint sensor of the type described above may, ifdesired, be incorporated within a touch-sensitive LCD panel, or on aflexible plastic substrate, using standard TFT technology, wherein theTFTs are placed in a two-dimensional arrayed formation. TheTFT/capacitive plate array is used to initially acquire signalstransmitted from the user's fingertip, and these signals are then passedon to a separate IC chip for processing to form an image of the user'sfingerprint. The pixel array may be provided in various sizes andconfigurations, including, but not limited to, round, square, andrectangular.

Another aspect of the present invention relates to a method of operatinga touch pad to create a fingerprint sensor for sensing a user'sfingerprint at substantially any location of the touch pad. In thisregard, a substrate is provided, the substrate having an upper surface.An array of finer-pitch pixels are formed upon the substrate andarranged along intersecting rows and columns. Each such finer-pitchpixel includes a switching device and a capacitive plate, and eachfiner-pitch pixel is spaced apart from adjacent finer-pitch pixels by afirst predetermined distance. A series of finer-pitch row address linesare provided to address the rows of finer-pitch pixels, each finer-pitchrow address line being associated with a row of finer-pitch pixels inthe array. Each finer-pitch row address line may be used to selectivelyaddress the finer-pitch pixels associated with each such finer-pitch rowaddress line. A series of finer-pitch column sensing lines are alsoprovided, each finer-pitch column sensing line being associated with acolumn of finer-pitch pixels for sensing a signal provided by thecapacitive plate of a finer-pitch pixel in an enabled row of the arrayof finer-pitch pixels.

The aforementioned method further includes the step of sub-dividing thefiner pitch pixels into a smaller array of “touch pad pixels”. The arrayof touch pad pixels is “smaller” in the sense that it has fewer rows andfewer columns, though it occupies the same two-dimensional space. Thesmaller array of touch pad pixels is also arranged in rows and columns.

Each touch pad pixel includes finer-pitch pixels located in at least twodifferent rows of the finer-pitch pixels, and also includes finer-pitchpixels located in at least two different columns of the finer pitchpixels. Each such “touch pad pixel” is spaced apart from an adjacenttouch pad pixel by a second predetermined distance that is at leasttwice as large as the first predetermined distance.

In a first mode of operation, the aforementioned method includes thestep of simultaneously enabling those finer-pitch pixels that aresub-divided into a common touch pad pixel for collective operation, andcollectively sensing signals provided by the capacitive plates of thefiner-pitch pixels grouped within the same touch pad pixel. In thismanner, each such touch pad pixel functions like a conventional sensorpixel in a typical touch pad. Using the sensed signals provided by thetouch pad pixels, a detection step is performed to detect whether apointer (e.g., a user's fingertip) is being applied to the upper surfaceof the substrate; if that is the case, the approximate location wheresuch pointer is being applied, relative to the substrate, is alsodetected.

In a second mode of operation corresponding to fingerprint sensing, themethod includes the step of determining which touch pad pixels lie nearthe detected pointer location. The finer-pitch pixels within each of thetouch pad pixels that lie near the pointer location are selectivelyswitched from collective operation to an individual operation mode.During the individual operation mode, the finer-pitch pixels in eachfiner-pitch row are individually enabled rather than beingsimultaneously enabled, and the signals provided by the capacitive plateof each finer-pitch pixel are individually sensed as each finer-pitchrow is enabled by a corresponding finer-pitch row address line. In thismanner, the finer-pitch pixels within the touch pad pixels that lie nearthe pointer location form a fingerprint sensor area for sensing afingerprint image of a user's fingertip.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is a cross-sectional diagram illustrating a first type of priorart fingerprint sensor wherein a pixel array is formed upon the surfaceof a silicon semiconductor chip, thus causing large silicon area to beconsumed.

FIG. 1b is a combined cross-sectional and top view of the prior artfingerprint sensor shown in FIG. 1a , and illustrating how an increasein pixel count directly increases silicon size.

FIG. 2a is a cross-sectional diagram illustrating another type of priorart fingerprint sensor wherein signal transmission electrodes and signalreceiving plates are formed by narrow metal traces located closelyproximate to each other.

FIG. 2b is a top view of the prior art fingerprint sensor shown in FIG.2a , and illustrates how the use of a an integrated circuit separatefrom the pixel array requires a large number of interconnecting metaltraces between the IC and the pixel array, which metal traces themselvesconsume a relatively large area.

FIG. 3a is a cross-sectional diagram showing one preferred embodiment ofthe present invention including a transmitter electrode ring surroundingthe periphery of a two-dimensional TFT/capacitive plate array formedupon a flex substrate, with a separate IC for signal processing.

FIG. 3b is a larger, partial view of the structure shown in FIG. 3 a.

FIG. 4a is a cross-sectional diagram illustrating one of theTFT/capacitive plate pixels within the pixel array.

FIG. 4b is a top view of four pixels within the pixel array.

FIG. 4c is a cut-away top view of the entire pixel array furtherillustrating the location of the transmission electrode ring, therow-address demultiplexer, and the column decode multiplexer.

FIG. 5 is an electrical schematic diagram of three rows, and fourcolumns, of the fingerprint sensor pixel array.

FIG. 6 is a partial schematic combining the sensing devices of FIG. 5with the related components of the fingerprint sensor, and illustratinghow the number of external signal paths to the IC are reduced.

FIG. 7a is a top view of the fingerprint sensor illustrating a metaltransmission ring placed around the sensor pixel array, along with metalpaths between the pixel array, transmitter electrode, and IC.

FIG. 7b is a partial top view of the upper left corner of FIG. 7a butincluding a detachable connector between the IC and the fingerprintsensor array.

FIG. 8 is an enlarged cross-sectional diagram similar to FIG. 3a , andillustrating how the signal radiated by the transmission electrode iscoupled through layers of the user's finger back to the ridges andvalleys of the user's fingertip into the pixel array.

FIG. 9 is a block diagram illustrating an alternate embodiment of theinvention wherein transmission electrodes are interspaced between rowaddressing electrodes of the pixel array.

FIG. 10 is a more detailed block diagram, similar to FIG. 9, but furtherillustrating a demultiplexing technique for driving the row addressingelectrodes.

FIG. 11 is a block diagram illustrating a portion of the row addressingelectrode demultiplexer, as well as the column decode multiplexer.

FIG. 12 is a timing diagram showing timing waveforms for signalsdepicted in FIGS. 9-11.

FIG. 13 is a top view of a fingerprint sensor area surrounded by atransmission electrode ring and adapted to be included as a portion of atouch-sensitive LCD panel.

FIG. 14 is a top view of a rectangular touch-sensitive LCD panel whereinthe fingerprint sensor area of FIG. 13 is incorporated within the lowerleft corner of the touch-sensitive LCD panel.

FIG. 15a is a top view of a touch-sensitive LCD panel generally similarto that shown in FIG. 14 but formed entirely of high-density pixelssub-divided into lower-density touch pad pixels.

FIG. 15b is an enlarged view of a portion of the touch-sensitive LCDpanel shown in FIG. 15a , and wherein a region contacted by a user'sfinger is re-configured as a higher-pixel density fingerprint sensorarea.

FIG. 16 is a simplified electrical schematic showing the manner in whichthe signals received by the TFT/capacitive plates of the pixel array aredifferentially sensed by differential amplifiers.

FIG. 17a illustrates the higher density pixels of the fingerprint sensorarea of the touch-sensitive LCD panel of FIG. 14 adjacent to lowerdensity pixel cells.

FIG. 17b is a view similar to that of FIG. 17a but illustrating how thehigher density pixels can be ganged together to simulate, or mimic, thefunction of the adjacent lower density pixel cells.

FIG. 18 is an enlarged view of four conventional pixel cells above, andfour ganged pixel cells formed by the higher density pixel cells below,along with corresponding metal electrodes.

FIG. 19 is a timing diagram showing timing waveforms for the signalsconducted by the electrodes shown in FIG. 18.

FIG. 20a is a top view of a fingerprint sensor constructed in accordancewith the present invention and embodied as a button.

FIG. 20b is a side view of the fingerprint sensor button shown in FIG.20 a.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following detailed description is to be read with reference to thefigures, in which like elements in different figures have like referencenumerals. The figures, which are not necessarily to scale, depictselected embodiments and are not intended to limit the scope of theinvention.

FIGS. 1a and 1b illustrate a known fingerprint sensor 30 which utilizesan entire silicon chip 32 as the receiver for signals derived from theuser's fingertip 34. Various methods have been described fortransmitting a signal into the user's finger, both on the silicon chipitself, and external to the silicon chip; in either case, the pixels 40used to receive signals from the user's fingertip are directly formed ona silicon chip. The signal is received at each pixel in the siliconchip. Signal detection may be based upon voltage amplitude, signal phaseshift, or other methods for detecting differences between signals thatpass through a ridge 36 of the fingertip versus signals that passthrough a valley 38 of fingertip 34. The primary disadvantage posed bythis approach is the cost, due to the size and complexity required forsilicon chip 32. Since the receiver pixels 40 are located in siliconchip 32 itself, a relatively large silicon area is consumed, and anyincrease in pixel count directly increases the required size of thesilicon chip. As the size of silicon chip 32 increases, the likelihoodof defects also increases; in some cases, even a single defect willrender the entire silicon chip 32 useless. In addition, this approachsuffers from a lack of flexibility in packaging methods, due to the needto protect the surface of silicon chip 32, which is similar to fragileglass.

FIGS. 2a and 2b illustrate another known fingerprint sensor 50 whereinan array of metal lines are arranged in an array of X rows 52 by Ycolumns 54. The series of X row lines 52 serve as receiver traces(“Rx”), and the series of Y column lines 54 serve as transmitter traces(“Tx”). The intersection of a Tx transmitter line and an Rx receiverline forms a single pixel area, which is designated 56 in FIG. 2b .Typically, each receiver line 52 is somewhat enlarged at each locationwhere it intersects a transmitter line 54 to form a “receiver plate”.Theoretically, a source signal travels from the TX transmitting line, upinto the user's fingertip 34, and back down to the Rx receiver line, inthe same location. A separate IC chip 58 is interconnected by additionalmetal traces 60 and 62 with the TX transmitter lines 54 and Rx receiverlines 52, respectively, for processing the image received by fingerprintsensor 50.

The known approach illustrated in FIGS. 2a and 2b suffers from severesignal-to-noise issues for multiple reasons, as will now be explained.Both the Tx transmitter lines 54 and the receiver “plates” provided ateach intersection of a Tx line 54 and receiver line 52 are formed withinpatterned metal layers. Due to relatively tight spacing within the pixelarray, each of the series of Tx lines 54 is relatively narrow, and eachof the receiver plates formed on receiver lines 52 is very small. Theeffective width of the Tx line is limited to the spacing betweenadjacent pixels. Moreover, the size of each receiver plate is alsolimited because the Tx lines 54 and Rx lines 52 must be formed onseparate metal layers (to avoid electrical shorts), and thereforeneither Tx line 54 nor the receiver plates of Rx line 52 can encompassthe entire pixel area; otherwise, the metal forming Tx line 54 wouldblock the transmitted signal from reaching a receiver plate locatedbelow Tx line 54. Alternatively, if the receiver plates were formed inan upper layer of metal, and the Tx lines 54 were formed in a lowerlayer of metal, then the receiver plates would block the transmittedsignal from reaching the user's fingertip. As a result, this approachsuffers from large signal losses, de-focusing of the transmitted signal,and thus poor image quality. In addition, IC chip 58 requires arelatively large number of I/O pads, directly related to the number ofTx transmit and Rx receive lines required. Routing of these signalsbetween IC chip 58 and the pixel array consumes a significantly largearea.

Further, the signal transmitted from Tx line 54, into fingertip 34, andback into the receiver plates formed on Rx line 52, cannot beeffectively isolated to the exact pixel area where such lines intersectwith each other. This is because the entire Tx line 54 is active, andthe entire Rx line is being sensed, at any given point in time. This cancause unwanted signals, which can vary depending upon the image beingacquired. For example, if a large finger ridge 36 is located over theentire Tx line 54, or over the entire Rx line 52, then the receivedsignal will be different than that from a similar pixel elsewhere whichonly has a ridge over the exact area of that one pixel, even though itis desired to detect equal signals from both pixels. This can causelarge image distortion issues, and complicate post-processingrequirements to attempt to reconstruct the proper image.

FIGS. 3a and 3b show a fingerprint sensor 70 in accordance with a firstembodiment of the present invention. A pixel array 72 includes an arrayof thin-film transistors (“TFTs”), or similar low cost switching devicesas an alternative to silicon integrated circuit devices. The TFTs may beformed upon a flexible substrate 73. Pixel array 72 is shown in greaterdetail in FIGS. 3b, 4a, 4b, 4c and 5 described below. Each pixel of thearray includes one TFT device. In addition, a relatively large metalcapacitor, or more accurately, “capacitive plate”, is formed at eachpixel within the array. A protective dielectric layer 78 is formed abovethe pixel array 72, and includes an uppermost surface 80 for receiving auser's fingertip 34. The capacitive plates are formed within flexiblesubstrate 73, below dielectric layer 78, but as near to the user'sfingertip as possible (generally near the uppermost surface upon which auser places his or her fingertip). Relatively wide Tx transmissiontraces 74 and 76 are shown in FIG. 3a extending along both sides ofpixel array 72 to transmit a high frequency signal from fingerprintsensor 70 into the user's fingertip 34, as indicated by arrows 82 and84, respectively. As will be explained in greater detail below, Txtransmission traces 74 and 76 may actually be portions of a single Txtransmission ring that encircles pixel array 72. Transmitter traces 74and 76 may be formed upon the lowermost surface of flexible substrate73. A separate integrated circuit silicon chip 86 is bonded to thelowermost surface of flexible substrate 73, and conductive traces extendalong and/or through flexible substrate 73 to interconnect IC chip 86and pixel array 72. While a flexible substrate 73 is illustrated, thedescribed structure can also be easily fabricated on glass or other morerigid substrates, as desired.

As shown best in FIGS. 4a and 5, each TFT 90 has a gate electrode 92coupled to a row address line 94. FIG. 5 is a schematic of the firstthree rows, and first four columns, of the pixel array 72. In FIG. 5,TFT 90 is located in the first row of the pixel array, and its gateelectrode 92 is coupled to row address line 94 for the first row (“Row1”) of pixel array 72. As indicated in FIG. 5, each row of pixel array72 (i.e., Row 1, Row 2, Row 3, . . . ) can be individually addressed,one at a time. This is accommodated by having a TFT (e.g. 90) at eachpixel location within pixel array 72. Apart from gate electrode 92, TFT90 also includes a drain electrode 96 and a source electrode 98,separated from each other by a semiconductor region 100. In addition,gate electrode 92 is spaced apart from semiconductor region 100, andfrom drain electrode 96 and a source electrode 98, by a gate dielectriclayer 102. When the row line 94 to which gate electrode 92 is coupled isselected, gate 92 renders semiconductor region 100 conductive, and drainelectrode 96 and source electrode 98 are electrically coupled to eachother. On the other hand, when row line 94 is not selected,semiconductor region 100 electrically isolates drain electrode 96 andsource electrode 98. The aforementioned construction of TFT 90 isentirely consistent with known methods currently used to produce TFTs onflexible substrates.

Still referring to FIGS. 4a and 5, a pixel electrode, or capacitiveplate, 104 is formed above TFT 90. Pixel electrode 104 extendssubstantially parallel to flexible substrate 73, and parallel to thesurface against which the user will ultimately press his or herfingertip against. Pixel electrode 104 is supported upon the uppermostsurface of an interlayer dielectric layer 106, which electricallyinsulates pixel electrode 104 from gate electrode 92 and row line 94. Avia 108 is formed through gate dielectric layer 102 and interlayerdielectric layer 106 to electrically couple pixel electrode 104 to drainelectrode 96 of TFT 90. As shown in FIG. 3b , when fabrication offingerprint sensor 70 is complete, the pixel electrodes 104, 110, 112,114, and 116 of pixel array 72 lie just below the user's fingertip,separated therefrom by a protective layer. It should be noted that thesource and drain lines of TFT 90 are located well below pixel electrode104, and do not enter into significant direct capacitive coupling withthe user's fingertip 34, thereby lessening any introduction of straysignals through the row lines or the column lines. When fabricated asdescribed above, the row lines are coupled to the gates of the TFTs, andthe column lines are coupled to the sources of the TFTs. Each pixelelectrode 104 forms a capacitor from the drain of each TFT to the user'sfingertip.

FIG. 3b illustrates pixel electrodes 104, 110, 112, 114, and 116 alllying in a common row of the pixel array. In reality, there are numerousrows of pixel electrodes. Turning briefly to the grid layout of FIG. 4b, pixel electrodes 104 and 110 lie adjacent each other in Row 1, and thegate electrodes of both TFT 90 and TFT 124 are commonly coupled to rowline 94. Two additional pixel electrodes, 118 and 120, are also shown inFIG. 4b , being located in the next succeeding row. The TFTs 126 and 128associated with pixel electrodes 118 and 120, respectively, have each oftheir gate electrodes coupled to row line 130, corresponding to Row 2 ofthe pixel array. Each pixel electrode essentially forms a capacitoralong with the user's fingertip and the protective dielectric layer thatelectrically separates the pixel electrode from the user's fingertip.The signal transmitted into the user's fingertip is coupled through eachsuch capacitor into pixel array 72. The plates of this effectivecapacitor are closer to each other when a ridge of the user's fingertipis directly above a pixel, while the plates of this effective capacitorare further from each other when a valley of the user's fingertip isdirectly above a pixel. Due to the variations in such capacitance frompixel to pixel, the transmitted signal coupled through the TFT of eachpixel will vary accordingly, and these variations can be used to formthe image of the fingerprint. In addition, by forming the large metalcapacitors, or “pixel electrodes”, near the top of the material stack,while placing the column lines further below, extraneous signals fromother locations in the array are more effectively isolated from the databeing detected by the column lines. Meanwhile, IC chip 86 can be mountedto the underside of flexible substrate 73 for driving the row lines, andprocessing the data signals provided by the column lines.

FIG. 4c is a simplified top view of fingerprint sensor 70. Pixel array72 includes X rows and Y columns of pixels, each such pixel beingfabricated in the manner described above in conjunction with FIGS. 4a,4b and 5. Pixel array 72 is surrounded, about its periphery, bytransmission ring 74′ for transmitting a high-frequency signal into theuser's fingertip. Transmission ring 74′ lies outside the area of pixelarray 72. Transmission ring 74′ effectively transmits a high frequencysignal into the body of the user's fingertip 34. This radiated signal isconducted by the user's fingertip, passes through the ridges and valleysof the fingertip surface, through a protective intervening material inthe customer's system (e.g., a cell phone cover glass), and is receivedonto the capacitive metal plates, or pixel electrodes, in pixel array72.

A demultiplexer 140 extends along one side of pixel array 72.Demultiplexer 140 receives control signals from IC 86 for indicatingwhich of the rows in the pixel array is to be selected at a given pointin time. Demultiplexer 140 decodes such control signals and drives therow lines (94, 130, . . . 142) to enable only one row at any given time.Demultiplexer 140 reduces the number of conductive traces that need toextend between IC 86 and pixel array 72. For example, if pixel array 72includes 256 rows of pixels, then pixel array 72 includes 256 row lines.On the other hand, there need only be eight binary control linesextending between IC 86 and demultiplexer 140 in order to select one ofthe 256 row lines.

Referring again to FIGS. 4b and 5, a series of Y column electrodes,including column electrodes 146, 148, 150 and 152, extends through pixelarray 72 to detect a signal derived from the pixel located within thecorresponding column of the selected row. For example, if row 94 isselected, column electrode 146, which is coupled to the source electrodeof TFT 90, senses the signal provided by pixel electrode 104; likewise,column electrode 148, which is coupled to the source electrode of TFT124, senses the signal provided by pixel electrode 110. Thus, the gateof each TFT is connected to a corresponding row driver, the drain ofeach TFT is connected to the pixel electrode (which forms a capacitorwith the user's fingertip), and the source terminal of each TFT iscoupled to a column line, or “data line”, to IC 86 for signalprocessing. The row lines (94, 130) and column lines (146, 148, 150 and152) shown can be formed of any conductive metal, including indium tinoxide (ITO), which is a transparent metal used in producing LCDs. Othermetals, such as aluminum or copper can also be utilized, whentransparency is not required.

Returning to FIG. 4c , column decode circuit block 144 is electricallycoupled to all of the column electrodes, including column electrodes146, 148, 150 and 152 shown in FIG. 5. Column decode circuit block 144thereby detects signal levels at each pixel in the selected row. Bystepping sequentially through the X rows of the pixel array, an image ofthe user's fingerprint may be derived having X by Y pixels. Thecapacitive plates, or pixel electrodes, can receive signal energy fromthe user's fingertip 34 in the form of a signal amplitude, a signalphase shift, or other methods, so long as the characteristics of thedetected signal are varied depending upon whether the transmitted signalpasses through the ridges, or the valleys, of the user's fingertip. Ifdesired, the characteristics (frequency, amplitude, etc.) of thetransmitted Tx signal can be varied from one detection pass through thepixel array to the next, and multiple samples can be taken and averagedto produce a more accurate image.

In use, a single row driver would be turned on, activating thatparticular row of pixels. Transmission ring 74′ is used to transmit aknown signal at a predetermined frequency. The signal contents of thepixels in the selected row is transferred onto the column lines fordetection, sensing, and processing within external IC chip 86. Thisapproach allows IC chip 86 to be physically displaced from the pixelarray, and allows for a reduction of row selection lines and sensedsignal data lines between IC chip 86 and the pixel array through the useof demultiplexing/multiplexing schemes. Each pixel electrode (104, 110,etc) forms a capacitor with the fingertip, and the value of thecapacitor will depend on the surface of the finger (ridge or valley)above each pixel location. Each pixel electrodes receives a signal fromthe fingertip, and if the associated TFT is “on” (i.e., the row driverfor this particular row is enabled to turn on the “gate” of this TFT),then the capacitively-coupled signal will be conducted through the TFT,and presented on the “data line”, or column electrode, of the pixelarray.

Column decode circuit block 144 is electrically coupled to IC 86, whichprocesses the signals detected at each pixel of the array to form theimage of the fingerprint. To minimize the number of conductive tracesbetween column decode circuit block 144 and IC chip 86, the detectedsignals may be transmitted from column decode circuit block 144 to ICchip 86 in blocks. For example, if pixel array 72 includes 256 columnsof pixels, column decode circuit block 144 could transmit blocks of 16signals at a time, requiring sixteen such transmissions for each row ofthe array. Apart from the 16 conductive traces between column decodecircuit block 144 and IC chip 86 for transmitting a block of signaldata, only four additional control lines might be needed to indicatewhich of the sixteen blocks of data is being transmitted. Thismultiplexing technique significantly reduces the number of metal tracesthat must extend between column decode circuit block 144 and IC chip 86.

FIG. 6 may help to illustrate further the manner in which demultiplexer140, and column decode circuit block 144 reduce the number of conductivetraces that need to extend between IC 86 and pixel array 72. Row lines94, 130, etc., are driven by demultiplexer 140. Demultiplexer 140 has anumber of output terminals equal to the number of row lines, and onlyone such row is selected at any given time. In contrast, the number ofselect lines 154 extending between IC 86 and demultiplexer 140 isexponentially reduced. Again, if there are 256 rows of pixels in thearray, then select lines 154 require only eight conductive traces touniquely identify one of the 256 rows. Thus, demultiplexer 140 can takea number (x) of select lines and address 2^(x) rows through simpledigital logic. Alternatively, demultiplexer 140 may simply be a digitalshift register wherein a logic “1” output signal is sequentially passedfrom the Row 1 output terminal, to the Row 2 output terminal, etc.,enabling one row at a time until all of the rows have been enabled.

Similarly, if the pixel array were to include 256 column electrodes,including column electrodes 146, 148, 150 and 152, and if the columnsare sub-divided into 16 blocks of 16 columns each, then column decodecircuit block 144 can transmit each block of data over sixteenconductive traces, represented in FIG. 6 by bus 156. Four additionalconductive traces, indicated in FIG. 6 by select lines 158, aresufficient to uniquely address one of the sixteen blocks of data for anygiven row of pixels, in this example. In the case of the column drivers,the signals will be analog, and must be de-muxed through analog muxes.As an example, it could be from 2^(Y) columns to one final analog input“M”, using Y select lines. Depending upon the configuration, M could be1 or a much larger number. This simply depends on the reduction ofsignals required, and the timing allowed for this process. For example,if the system chooses to address a single pixel at a time (M=1), thenall X by Y pixels must be addressed individually.

FIG. 7a better illustrates the location of the Tx transmission ring 74′relative to the pixel array 72. Tx transmission ring 74′ can be a simplemetal structure, and could be formed either above the gate dielectriclayer 102 (see FIG. 4a ) or below gate dielectric layer 102. Txtransmission ring 74′ can either be driven with a signal sourced by ICchip 86, or by an external driver. Tx transmission ring 74′ is placedsufficiently far enough away from pixel array 72 so as to avoid directsignal injection from transmission ring 74′ to the pixel electrodes;rather, the high frequency signal radiating from transmission ring 74′must first pass into the user's fingertip 34 before being coupled to thepixel electrodes. Within FIG. 7a , connector 160 extends between pixelarray 72 and IC chip 86 for passing electrical signals therebetween. Ifdesired connector 160 could be formed as a detachable connection 160′ asshown in FIG. 7 b.

FIG. 8 is similar to FIG. 3a already described above, but betterillustrates how the signal radiated by transmission electrodes 74 and 76is coupled through layers of the user's finger back to the ridges andvalleys of the user's fingertip and into the pixel array. In FIG. 8, theuser's fingertip is designated generally by reference numeral 170. Theexternal layer of the user's fingertip, which includes ridges andvalleys, is designated 172. Directly above external layer 172 is aninner conductive layer of tissue 174. The high-frequency signal radiatedfrom transmission electrodes 74 and 76 passes upwardly along arrows 82and 84, respectively, through the protective dielectric layer 78,through external layer 172 of the user's fingertip, and into the moreconductive layer of tissue 174. The signal conducted by layer 174 isradiated back downwardly toward pixel array 72, through the ridges andvalleys of external layer 172, and through protective dielectric layer78 along the path indicated by arrows 176, for reception by the pixelelectrodes (designated 178 in FIG. 8). Within FIG. 8, the metal layer180 disposed below pixel array 72 represents, for example, the routingof the row address lines used to select the active row in the pixelarray. Once again, IC chip 86 may be mounted to the underside of thesubstrate supporting pixel array 72.

Within FIG. 9, an alternate embodiment of the invention is illustratedin block diagram form wherein transmission electrodes are interspersedbetween row addressing electrodes of the pixel array. In this example,the pixel array 72 is a matrix of 96 rows and 96 columns. As in thepreviously described embodiment, pixel array 72 is formed by a matrix ofpixel cells, each pixel cell including a TFT and a pixel electrode. InFIG. 9, block 190 represents the logic used to address the row lines 192(row G0), 194 (row G1), through 196 (row G95). Row line 192 (G0) iscoupled to the gate electrodes of the TFTs lying in a common first row(note that, in FIG. 9, the array has been turned ninety degrees, and therows actually extend up and down the page). Likewise, row line 194 (G1)is coupled to the gate electrodes of the TFTs lying in a common secondrow. Row line 196 (G95) is coupled to the gate electrodes of the TFTslying in a common last row of pixel array 72.

As in the case of the previously described embodiment, the column, ordata, electrodes, including lines 198 (C0), 200 (C1), through 202 (C95),are coupled to the source terminals of the TFTs lying along a commoncolumn in order to sense the signal received by the pixel electrode inthe addressed row of pixels. Column electrode lines 198 (C0), 200 (C1),through 202 (C95) are each coupled to column decode circuit block 144for detecting the signals received by each pixel electrode within theselected row of pixels.

Within the embodiment shown in FIG. 9, there are also a series oftransmission electrodes, including TX0 line 204, TX1 line, . . . , TX10line 206, through TX95 line 208, which also extend from TFT row logicblock 190, and which extend in alternating fashion between the rowaddressing lines 192 (row G0), 194 (row G1), through 196 (row G95).These TX lines can be used, in lieu of the transmission ring 74′ shownin FIG. 7a , to transmit the high-frequency signal into the user'sfingertip. At first, this might seem to contradict Applicants' objectiveof keeping the transmission electrodes laterally spaced apart from thepixel array to prevent direct capacitive coupling from the transmissionelectrodes to the pixel electrodes (i.e., bypassing the user'sfingertip). However, by judiciously selecting which ones of thetransmission electrodes are active at any given time, and by keeping theactive transmission electrodes away from the selected row of the pixelarray, one can largely avoid the problem direct capacitive coupling fromthe transmission electrodes to the pixel electrode. In addition, byplacing the transmission electrodes within the pixel array, rather thanaround it, one avoids the need to dedicate extra area on the substratefor a transmission electrode that encircles the perimeter of the pixelarray.

FIG. 10 is a higher level functional diagram of logic components formedwithin TFT row logic block 190 for creating the driving signals used tocontrol row addressing lines 192 (row G0), 194 (row G1), through 196(row G95), and TX0 line 204, TX1 line 206, . . . , TX10 line 207,through TX95 line 208. The operation of the TFT row logic block 190 willbe better understood by reference to the timing waveforms shown in FIG.12. IC chip 86 sends an initiation signal 220 to TFT row logic block 190to commence a read cycle of the pixel array 72, and clears/resets logicelements. Pulsed clock signal 222 is also provided by IC chip 86 to TFTrow logic block 190 to provide a timing reference. IC chip 86 also sendsa pulse GD on line 224 to indicate that the sequence of enabling the rowaddressing lines is to begin. On the first clock cycle C when pulse GDis active, row address line 192 (G0) goes active to enable the gateelectrodes of the TFTs in the first row of pixels. On the nextsuccessive clock cycle C, row address line 192 (G0) switches back low,while the next row address line 194 (G1) goes active. This processcontinues until, on the 96th clock cycle, row address line 196 (G95)goes active.

Still referring to FIGS. 9, 10 and 12, Tx signal 226 represents the highfrequency signal to be transmitted into the user's fingertip. As statedabove, Tx signal 226 may be provided by IC chip 86, or from an externalsource. A further signal TD is provided on line 228 as an input to afirst flip-flop register 230. The output of flip-flop register 230 iscoupled to an AND gate 232. When the output of flip-flop register 230 islow, AND gate 232 blocks signal TX on line 226 from being transmitted toTX0 line 204. The output of flip-flop register 230 also serves as thedata input to a next successive flip-flop register which likewisecontrols an AND gate driving TX1 line 206, and so forth.

In regard to FIGS. 9 and 10, Applicants have determined that it issatisfactory to have approximately ten rows of inactive transmitterlines on each side of the active row of the pixel array being sensed atany given time. Thus, if the first row of pixels is being selected bygate electrode 192, it is then desired to disable the first tentransmission electrodes 204 (TX0), 206 (TX1), . . . through TX9.Initially, the first eleven flip-flops, including first flip-flop 230,are reset by the Initialize (I) signal 220, while the remainingflip-flops are set by the Initialize (I) signal 220. Thus, when thefirst row of pixels is selected, the first eleven transmissionelectrodes 204 (TX0), 206 (TX1), . . . through TX10 are disabled bytheir respective AND gates (including AND gate 232), while TX11 throughTX95 are enabled, for a total of 85 active transmission electrodesinitially.

For each succeeding clock cycle, another transmission electrode isdisabled, until the number of active transmission electrodes decreasesto 76. During the first ten clock cycles of clock signal 222 (C), the TDinput signal 228, provided by IC chip 86, is maintained at a logic lowlevel (“0”) to keep the first eleven transmission electrodes 204 (TX0),206 (TX1), . . . through TX10 disabled by their respective AND gates,while one additional transmission electrode to the right becomesdisabled. After ten such clock cycles, TD input signal 228 switches to alogic high level (“1”), and remains high for the remainder of the clockcycles used to finish reading each of the rows of pixels. Thus, as theeleventh row of pixels is selected by row address line G10, transmissionelectrode 204 (TX0) is enabled to transmit the high-frequency signalinto the user's finger, while neighboring transmission electrodes TX1through TX20 are disabled. This pattern is continued on each clockcycle, effectively providing ten inactive TX rows on either side of theselected row being sensed. As the active sensing row moves across thepixel array with each clock cycle, so does the “inactive range” of theTX transmission electrodes. As the last twenty rows are selected forsensing, the number of inactive transmission electrodes begins todecrease from twenty down to ten. In the example shown, the maximumnumber of TX transmission electrodes that are disabled at any one timeis twenty, but in that instance, at least 76 other TX transmissionelectrodes are enabled at the same time to reliably transmit the highfrequency TX signal into the user's finger.

As shown in FIG. 12, during the time that row address line 192 (G0) isselected, transmission lines 204 (TX0), 206 (TX1) through 207 (TX10)remain low, or disabled. On the other hand, transmission lines 234(TX11) through 208 (TX95) are actively driven with the high frequency TXsignal to help transmit the TX signal into the user's fingertip.Similarly, when the row address line/gate electrode G10 is selected, thetransmission electrodes for TX1 through TX10, and TX11 through TX20, aredisabled, while all the other TX lines (i.e., TX0 and TX21-TX95) areenabled to help transmit the TX signal into the user's fingertip. Inthis manner, the pixel electrodes that are being sensed do not receivethe transmitted TX signal directly from neighboring TX lines, but onlyas a result of the capacitive coupling between the pixel electrodes andthe user's fingertip. Moreover, all of the logic circuitry shown in FIG.10 for TFT row logic block 190 can be formed upon the same flexiblesubstrate in which pixel array 72 is formed, using the same types ofTFTs used to form the pixel array.

FIG. 11, together with the waveform diagram of FIG. 12, illustrates themanner in which the column decode circuitry 144 (see FIGS. 9 and 10) canbe constructed as a multiplexer to reduce the number of conductive linesrunning between pixel array 72 and IC chip 86. Three binary selectionsignals, represented by bus 250, are received by a 3-to-8 decoder 252,which provides eight output lines, including lines 254 through 256. Afirst multiplexer, shown in dashed box 258, includes a series of eightTFTs. The first such TFT extends between column electrode 260 (c95) andoutput port 264 (Cout11). The last of such TFTs in dashed box 258extends between column electrode 262 (c88) and output port 264 (Cout11).Thus, at any given time, one of the eight column electrodes c95 throughc88 is coupled to output port Cout11.

Still referring to FIG. 11, there are eleven more such multiplexersprovided, including those shown as 266 and 268, for providing datasignals on output ports 270 (Cout0) through 272 (Cout10) in the samemanner. Thus, at any given time, twelve output signals are provided viaoutput ports 270 through 264 (Cout0 through Cout11). By cycling theselection bus signals 250 through each of their eight possible states,all 96 columns of data can be sensed and conducted to IC chip 86. Asindicated in FIG. 12, the selection bus signals 250 are cycled throughtheir eight possible states during each row address period so that allcolumns in a selected row can be sensed. Thus, in FIG. 12, whenselection bus signals 250 are in a first state (=0), column c0 issensed, and the transmitted TX signal waveform is recreated on line 198(c0), to a greater or lesser extent, depending upon whether a ridge orvalley of the user's fingertip lies above the corresponding pixelelectrode. Similarly, when selection bus signals 250 are in a secondstate (=1), column c1 is sensed, and the transmitted TX signal waveformis recreated on line 200 (c1), to a greater or lesser extent, dependingupon whether a ridge or valley of the user's fingertip lies above thecorresponding pixel electrode.

Those skilled in the art will appreciate that the fingerprint sensorsdescribed above may be incorporated within a conventional LCD touchpadof the type used in a computer with a touch screen monitor, a computertablet, or a cell phone. For example, FIG. 13 illustrates a fingerprintsensor 300 of the general type described above in conjunction with FIGS.3-8, including a transmission electrode ring 302 surrounding a pixelarray 304. Turning to FIG. 14, cell phone LCD touchpad display panel 310incorporates fingerprint sensor 300, of FIG. 13, within the lower leftcorner thereof.

A typical LCD touchpad, i.e., the portion of panel 310 of FIG. 14 lyingoutside fingerprint sensor area 300 is formed of a two-dimensional arrayof relatively low density pixels, arranged on a pitch of approximately500 microns, i.e., the distance from the center of one pixel cell to thecenter of the next pixel cell is approximately 500 microns. The pixeldensity can be relatively low because the panel merely needs to detectthat a fingertip, or stylus, is in contact with a region of displaypanel 310. In contrast, to function properly, fingerprint sensor arrayportion 300 should have higher density pixels, arranged on a much finerpitch of approximately 50-70 microns center-to-center. Thus, fingerprintsensor area 300 may have as many as ten rows of pixels for every pixelrow in touchpad panel 310. Likewise, fingerprint sensor area 300 mayhave as many as ten columns of pixels for every pixel column in touchpadpanel 310.

However, the technology used to fabricate fingerprint sensor area 300 isvery similar to the technology used to fabricate the low density,touch-sensitive pixels throughout the remainder of display panel 310.Thus, while fingerprint sensor area 300 is shown in FIG. 14 as beinglimited to the lower left corner of display panel 310, it should beunderstood that fingerprint sensor area 300 could be expanded to coverthe entire bottom of display panel 310, if desired, or any other areawithin display panel 310.

To further guard against extraneous noise signals, the accuracy ofsensing the signals detected by the pixel electrodes may be furtherenhanced by sensing the pixel electrode signals in a differential mode.This approach allows for the removal of all types of common mode noise,whether originating from the human body itself, or from other sourcessuch as the electronic equipment in which the fingerprint sensor ishoused. Referring to FIG. 16, TFT 400 and its associated pixel electrode401 form a first pixel of a fingerprint sensor area; TFT 402 and itsassociated pixel electrode 403 form a second pixel within the same rowat TFT 400; and TFT 404 and its associated pixel electrode 405 form athird pixel within the same column as TFT 400. TFT 400 and TFT 402 arecoupled to a first row addressing line 406. TFT 404 is coupled to asecond row addressing line 408. The source terminals of TFT 400 and TFT404 are both coupled to a first column electrode 410; the sourceterminal of TFT 402 is coupled to a second column electrode 412.

Still referring to FIG. 16, first column electrode 410 is coupled to apositive (non-inverting) input of a first differential amplifier 414.Similarly, second column electrode 412 is coupled to a positive(non-inverting) input of a second differential amplifier 416. Similardifferential amplifiers are provided for the remaining column electrodesof the pixel array, except that, in one instance, as will be explainedbelow, the number of such differential amplifiers may be one less thanthe number of column electrodes. Column electrode 418 is coupled to thenegative (inverting) terminal of each of the differential amplifiers414, 416, etc. Column electrode 418 runs the length of the pixel arrayto the same extent as typical column electrodes 410, 412, etc. Anystray, interfering signals that are received by typical columnelectrodes (e.g., 410 and 412) are also received by column electrode418. Thus, differential amplifier 414 will effectively subtract thestray, interfering signal presented by column electrode 418 from thesignal presented by column electrode 410, and the resulting outputsignal 420 will include the signal sensed by the pixel electrode in theselected pixel row, without unwanted noise components.

Within FIG. 16, column electrode 418 may be a “dummy” electrode that isnot actually coupled to any active pixels, if desired. For best noiserejection of human body noise, this dummy column should be located in anarea that is equally proximate to the user's finger as the actual columnelectrodes. Alternatively, column electrode 418 may be an actual columnelectrode associated with a particular column of pixels in the array; inthat case, there will be no sensed data for the column electrode thatwas “sacrificed” to receive common mode noise. In order to recreate datasignals for the missing column of the pixel array, it should be realizedthat fingerprints are not arbitrary data. By definition, a fingerprintincludes ridges (maximums) and valleys (minimums), and a fingerprintimage ranges between such minimum and maximum signals. One relativelysimple method for reconstructing the data in the missing column of thepixel array is to compute the missing data value by looking at thevalues of the surrounding pixels, and interpolating the value in eachrow of the missing column.

It has been mentioned above, relative to FIGS. 13 and 14, thatfingerprint sensor area 300 may be incorporated within a conventionalLCD touchpad panel 310. To further aid in the integration of fingerprintsensor area 300 within a conventional LCD touchpad panel, fingerprintsensor area 300 may be selectively operated in a mode that mimics theoperation of the surrounding lower density sense pixels.

Referring to FIG. 17a , fingerprint sensor area 500 is formed in thelower left corner of touchpad display panel 502. In this example, thesurrounding transmission ring 302 of FIG. 13 is omitted in favor ofinterleaved transmission electrodes operated in the general mannerdescribed above in conjunction with FIGS. 9-12. Touchpad display panel502 includes an array of low density sense pixels 504 through 520. Eachsense pixel 504 through 520 serves to detect whether a user's fingertip,or a stylus, is being placed against a cover glass directly above suchsense pixel. In the example shown in FIG. 17a , there are approximatelyseven high-density pixels, per linear length, in fingerprint sensor area500 for every single low density pixel (e.g., 504). The region occupiedby fingerprint sensor area 500 replaces what would otherwise have been afour-by-four array of low density pixels. When fingerprint sensor area500 is being used in “fingerprint mode” to generate a fingerprint image,each pixel electrode in the pixel array is individually addressed andsensed in the manner already described above.

Turning to FIG. 17b , during the “mimic mode” of operation, groupings ofseven-by-seven pixels within fingerprint sensor area 500 are gangedtogether to form virtual low density pixels, to mimic the low densitypixels 504-520 of touchpad panel 502. As shown in FIG. 17b , virtual lowdensity pixel 522 is formed by a seven-by-seven array of high densitypixels. When FPS area 500 is configured to mimic the touch pad area 502,the rows of FPS area 500 are selected, or de-selected, in a ganged mode.The number of rows in FPS area 500 that are enabled/selected at any onetime depends on the size of the higher density FPS pixels versus thesize of the lower density touchpad pixels. In the example shown in FIGS.17a and 17 b, groups of seven row of FPS area 500 are ganged together.Apart from selecting seven adjacent rows simultaneously, seven adjacentcolumn electrodes are also shorted together. In this manner, the signalscollected on the pixel electrodes of 49 pixels (7×7) are averagedtogether, providing one output signal that mimics signals produced byeach of the lower density pixels (e.g., 504, 506, etc.) of touchpaddisplay panel 502. Those skilled in the art will appreciate that, inthis “mimic mode” of operation, it would be possible, if desired, tocouple the ganged/shorted column electrodes in FPS area 500 with theexisting column lines of the touchpad panel for sensing and processingof the signals provided thereby.

As shown in FIG. 18, four low density pixels of touch pad 502 include503, 504, 505 and 506. These low density sensor pixels are addressed byrow lines 530 and 532, and by column lines 534 and 536. In FIG. 18, thehigher density pixels of fingerprint sensor area are shown gangedtogether, this time at a higher density of ten pixels by ten pixels pervirtual low density pixel. In this example, the touchpad pixels (503,504, 505, 506) are 500 um×500 um, while the FPS pixels are 50 um×50 um.In order for the FPS area to mimic the touchpad area, FPS Row 1 throughFPS Row 10 are turned “ON” at the same time, and the FPS columns areshorted together in groups of 10 columns (i.e., FPS Column 1 through FPSColumn 10 shorted; FPS Column 11 through FPS Column 20 shorted; . . .etc. . . . ). In this manner, the virtual FPS pixel size can beincreased from 50 um×50 um to 500 um×500 um. Ganging the FPS rowstogether by selecting multiple rows at the same time, and the shortingof the desired number of columns together, is accomplished using theTFTs. This method allows a fine pixel FPS to mimic the large pixel ofthe touch pad.

The higher density pixels of fingerprint sensor area 500 are addressedby row selection lines 548, and the columns of the selected rows aresensed by column electrode lines 550. Four virtual low density pixelsare designated 540, 542, 544 and 546. Ten row address lines (FPS Row 11through FPS Row 20) can individually address each of the ten rows(11-20) within virtual low density pixel 540 when a fingerprint image isrequired, or all ten of such rows can be selected at the same time whenmimicking the operation of the lower density pixels. Similarly, tencolumn sensing electrodes (FPS Column 1 through FPS Column 10) canindividually sense each of the ten columns (1-10) within virtual lowdensity pixel 540 when a fingerprint image is required, or all ten ofsuch columns can be shorted together when mimicking the operation of thelower density pixels.

The timing waveform of FIG. 19 further illustrates the operation of thefingerprint sensor area (FPS) 500, and its higher density pixels, whenbeing used to mimic the operation of the lower density pixels. During afirst clock cycle (t0), FPS Rows 1-10 are all enabled so that virtualpixels 544 and 546 can be selected. During that same clock cycle, FPSColumns 1-10 are shorted to sense virtual pixel 544, and FPS Columns11-20 are shorted to sense virtual pixel 546. During the next clockcycle, (t1), FPS Rows 1-10 are all disabled once more, while FPS Rows11-20 are all enabled so that virtual pixels 540 and 542 can beselected. Once again, during clock cycle t1, FPS Columns 1-10 areshorted to sense virtual pixel 540, and FPS Columns 11-20 are shorted tosense virtual pixel 542. This process continues until all virtual pixelshave been sensed. Thereafter, a sufficient number of additional clockcycles follow during which the rows of the touchpad panel (i.e., TP Row1; TP Row 2; TP Row 3; . . . TP Row N), including rows 530 and 532, areindividually selected, and touchpad column lines (e.g., 534 and 536) aresensed to determine whether any of the pixels in the selected touchpadrow are being touched by a user's finger or a stylus.

Reference is now made to FIGS. 14 and 15 b which illustrate an LCDtouch-sensitive panel, or touch pad, which is formed of a high-densityTFT/capacitive plate pixel array, but which is usually operated in alower-density touch pad mode. When operated in touch pad mode, thehigher density (finer-pitch) pixels mimic the operation of lower densitytouch pad pixels in the manner already explained above. FIGS. 15a and15b also aid in illustrating a method of operating such a touch padwherein a fingerprint sensor area can be selectively provided atwhichever location of the touch pad that a user happens to be applyinghis or her finger. In FIG. 15a , touch pad 702 includes an array oflow-density touch pad pixels, including adjacent touch pad pixels 704and 706. In FIG. 15a , a user's fingertip is being applied against a“contact” region, namely, the area of the touch pad designated by dashedoval 700.

FIG. 15b is an enlargement of the lower half of touch pad 702, anddashed oval 700 is shown superimposed over a rectangular region of touchpad 702 formed by touch pad pixels that lie near dashed oval 700,including the areas corresponding to touch pad pixels 704 and 706.However, in FIG. 15b , touch pad pixels 704 and 706, and other touch padpixels lying near dashed oval 700 have been switched to fingerprintsensor mode, wherein the finer-pitch, high-density pixels are nowoperated individually, rather than collectively, to provide higherresolution than when operated in touch pad pixel mode.

The ability of touch pad 702 to mimic a lower density touch pad enablesthe entire touch panel area, or any portion of the touch panel area, tobe utilized as a fingerprint sensor area. A finger print image can betaken from any location on touch panel 702, if desired. Initially, touchpad 702 can be configured as a conventional touch pad by shortingaddressing rows together, and shorting column sensing lines together, toform larger, less dense pixels, as described above. Alternately, touchpanel 702 could be used as one large, high-resolution fingerprintsensor, in which one finger, multiple fingers, or even portions of aperson's palm, could be sensed. Lastly, touch panel 702 could beconfigured as a touch pad to first detect where a person's fingertip islocated, and then, dependent upon the fingertip location, configure thedetected location as a fingerprint sensor area to image the person'sfingerprint.

Whether touch panel 702 is being used as a touch pad or as a fingerprintsensor area, the general location at which the user is applying his orher fingertip can be detected. When touch panel 702 mimics the operationof a conventional touch pad, the same methods currently used todetermine finger location, and finger swipe direction, for conventionaltouch pads may also be used for the same purposes for touch panel 702.Once the location of the fingertip is determined, the immediatelysurrounding region is converted to a high-resolution fingerprint sensorarea for allowing a high quality image of a fingerprint to be captured.

In practicing the method illustrated by FIGS. 15a and 15b , an array offiner-pitch pixels are arranged along intersecting rows and columnsessentially across the entire surface of the touch panel 702. Asdescribed earlier, each finer-pitch pixel preferably includes aswitching device (e.g., a TFT) and a capacitive plate for sensing ahigh-frequency signal transmitted through the user's finger. A series offiner-pitch row address lines are provided, each being associated with arow of finer-pitch pixels in the array. Each finer-pitch row addressline can selectively address the finer-pitch pixels associated with thecorresponding row of the array. As before, a plurality of finer-pitchcolumn sensing lines are also provided, each finer-pitch column sensingline being associated with a column of finer-pitch pixels in thefiner-pitch pixel array. Each such finer-pitch column sensing line cansense a signal provided by the capacitive plate of a finer-pitch pixelin a selected row of the finer-pitch pixel array. While not shown inFIG. 15a or 15 b, those skilled in the art will understand thathigh-frequency signal transmitter electrodes, as described aboverelative to FIGS. 9 and 10, can be included in the pixel array, and canbe selectively enabled and disabled, to effectively transmit thehigh-frequency signal into a user's fingertip applied to the “contact”region of touch pad 702.

As mentioned above in regard to FIGS. 17a, 17b , and 18, the finer-pitchpixels are grouped, or sub-divided, into an array of larger touch padpixels, like touch pad pixels 704 and 706 in FIG. 15b , which are alsoarranged in rows and columns. The array of touch pad pixels is a“smaller array” in the sense that it includes fewer rows and columns,though it covers the same basic two-dimensional space. Each such touchpad pixel includes finer-pitch pixels located in at least two, andtypically five or more, different rows of the finer-pitch pixels.Likewise, each of the touch pad pixels includes finer-pitch pixelslocated in at least two, and typically five or more, different columnsof the finer pitch pixels. The center-to-center distance from one suchtouch pad pixel to the next adjacent touch pad pixel is at least twiceas large, and more typically, five or more times larger than thecenter-to-center distance from one finer-pitch pixel to the nextadjacent finer-pitch pixel.

Still referring to FIGS. 15a and 15b , the aforementioned methodincludes a first mode of operation in which all of the finer-pitchpixels within touch pad 702 are configured to mimic the operation oflower density touch pad pixels, like 704 and 706. In this first mode ofoperation, the finer-pitch pixels that collectively form touch pad pixel704, for example, are simultaneously enabled for collective operation.In other words, all of the row address lines for the finer-pitch pixelslocated in touch pad pixel 704 are enabled at the same time (i.e, therow addressing lines that enable that group of finer-pitch pixels areeffectively shorted together). Likewise, the column sensing electrodescoupled to the finer-pitch pixels located in touch pad pixel 704 arealso effectively shorted together for collectively sensing the signalsprovided by the capacitive plates of all of the finer-pitch pixelslocated within touch pad pixel 704. In this manner, each such touch padpixel mimics the function of a conventional touch pad pixel of aconventional touch pad during the first mode of operation. During suchfirst mode of operation, signals provided by the touch pad pixels areused to detect whether a pointer (a fingertip, a stylus, etc.) is beingapplied proximate to the upper surface of touch pad 702, and if so, thelocation of such pointer on touch pad 702 is also detected.

Referring to FIG. 15b , touch pad 702 can be changed to a second mode ofoperation when, for example, an image of a user's fingertip is to beimaged. During this second mode of operation, it is determined whichtouch pad pixels lie proximate to the pointer location. For example, inFIG. 15b , a group of 20 touch pad pixels (extending four touch padpixels across, and five touch pad pixels tall) are determined to lieproximate to the pointer location; among those 20 touch pad pixels arethose designated 704 and 706 in FIG. 15a . Those 20 touch pad pixels arethen re-configured, i.e., selectively switched, back to the finer-pitchpixels that can be individually addressed and sensed. During thisindividual mode of operation, the finer-pitch pixels in each finer-pitchrow are individually enabled, and the signal provided by each capacitiveplate of each such finer-pitch pixel is individually sensed as eachfiner-pitch row is enabled by a corresponding finer-pitch row addressline. This is why, in FIG. 15b , the array, or grid illustrated in“contact” region 700 is drawn as being much more dense then the touchpad pixels located outside “contact” region 700. Accordingly, theindividual finer-pitch pixels within “contact” region 700 are nowconfigured to form a fingerprint sensor area for sensing a fingerprintimage of a user's fingertip.

FIGS. 15a and 15b illustrate the manner in which touch pad 702 canprovide a fingerprint sensor area proximate to a contact regioncurrently being touched by a user's finger, anywhere within touch panel702. Those skilled in the art will appreciate, however, that if desired,the fingerprint sensor area could be expanded to encompass the entiresurface of touch pad 702. In other words, every touch pad pixel shown inFIG. 15a could be switched from its first (or “mimic”) mode of operationto its second (or “individual”) mode of operation, resulting in theentire touch pad surface being operated in high-density pixel mode. Inthis manner, two or more fingerprints, or even a palm print, image couldbe sensed.

A fingerprint sensor of the type generally described above may also beincorporated within a button, as illustrated in FIGS. 20a and 20b .Button 600 may, for example, be a security feature on a protecteddoorway, an elevator, etc., wherein admission is limited to those whosefingerprint matches a stored fingerprint of an individual whose identityhas previously been verified. Button 600 includes a pixel array 602covered by a protective layer of glass 604. The pixel array is formedupon a flexible substrate 606 mounted to a printed circuit board 608 byan adhesive 609. An integrated circuit chip 686 is mounted to printedcircuit board 608. A flexible electrical connector 610 extends betweenflexible substrate 606 and printed circuit board 608 for makingelectrical connections between pixel array 602 and IC chip 686. WithinFIG. 20a , surrounding ring 612 may be provided to transmit the highfrequency signal into the user's finger as the user depresses the buttonwith his or her fingertip. Alternatively, the outer transmissionelectrode may be omitted, if desired, and transmission electrodes may beselectively activated, away from the particular row being sensed at agiven moment, in the manner already described above.

Those skilled in the art will appreciate that the fingerprint sensordescribed herein lends itself to applications for touch electronics, andincreases the ability of the sensor to read fingerprint images throughthicker protective surfaces that provide greater protection to theunderlying pixel array. Thus, for example, the present fingerprintsensor can be used successfully with thick cover glasses often providedin cell phone touch display panels. In addition, the apparatus describedherein can be used with a relatively small integrated circuit chip,independent of the dimensions of the pixel array, which reduces cost ofmanufacture. The disclosed fingerprint sensor provides an increase insignal strength, and improved signal-to-noise ratio through thickmaterials, and the ability to keep the signal “focused” through thickermaterials, so that the image of the finger ridges and valleys is at ahigh enough resolution for proper detection. The invention describedherein can be used in any application which uses a touch sensitivesurface such as cell phones, touch pads, notebooks, notepads, E-readers,and the like. The invention can be used to embed biometric securitywithin electronic products with minimal impact to product size, cost,and processing.

From the foregoing description of the preferred embodiments, thoseskilled in the art will recognize that the present invention provides afingerprint sensor for imaging a person's fingerprint without requiringthe use of an integrated circuit semiconductor chip of the samedimensions as the pixel array used to capture the image of thefingerprint, thereby significantly lowering production costs. Thefingerprint sensor of the present invention more readily distinguishesthe ridges and valleys of a fingertip applied to a cover plate overlyingthe pixel array, even with relatively thick cover plates. The disclosedinvention effectively transmits a high frequency carrier signal into theperson's fingertip without simultaneously coupling such carrier signaldirectly into the pixel array, i.e., the carrier signal is forced topass into the user's fingertip before being transmitted back into thepixel array.

As noted above, a fingerprint sensor constructed in accordance with thepresent invention also significantly reduces the number of electricallines which must extend between the pixel array and an associatedintegrated circuit, used to process the fingerprint image captured bythe pixel array, largely by demultiplexing the row address lines and/orby multiplexing the column data lines. The present invention lendsitself to differential sensing of the signals detected by the pixelelectrodes, resulting in improved rejection of common mode noisesignals. A fingerprint sensor of the present invention can easily beincorporated within a conventional touch-sensitive pad, and can evenmimic the lower density pixels of the touchpad when not being used toform an image of a fingerprint, while using the same sensing layers, andsame fabrication techniques.

While the present invention has been described with respect to preferredembodiments thereof, such description is for illustrative purposes only,and is not to be construed as limiting the scope of the invention.Various modifications and changes may be made to the describedembodiments by those skilled in the art without departing from the truespirit and scope of the invention as defined by the appended claims.

We claim:
 1. A fingerprint sensor, comprising: a first substrate havinga plurality of pixels formed therein, the pixels being arranged in apixel array of R rows and N columns, each pixel comprising a switchingdevice and a capacitive plate; a plurality of R row addressingelectrodes, each such row addressing electrode being coupled to theswitching devices of the pixels in a corresponding row of the pixelarray for selectively enabling the switching devices in thecorresponding pixel row; a plurality of N data electrodes, each suchdata electrode being coupled to the switching devices of the pixels in acorresponding column of the pixel array for sensing a signal provided bythe capacitive plate of the pixel located at an intersection of aselected pixel row and the corresponding column of the pixel array; acommon electrode extending at least partially through the pixel array onthe first substrate and arranged between the two adjacent dataelectrodes; a cover layer overlying the first substrate for receiving afingertip of a user; and a plurality of differential amplifiers, eachdifferential amplifier having a first input coupled to one of theplurality of data electrodes and having a second input coupled to thecommon electrode, each differential amplifier having an output terminalfor providing an output signal, the output signal being representativeof the difference between a signal provided by a data electrode and asignal provided by the common electrode.
 2. The fingerprint sensor asclaimed in claim 1, wherein the common electrode is parallel with thedata electrodes in the pixel array.
 3. The fingerprint sensor as claimedin claim 1, wherein the first input of the differential amplifier is anon-inverting input, and the second input of the differential amplifieris an inverting input.
 4. The fingerprint sensor as claimed in claim 1,wherein the common electrode is a dummy electrode.
 5. The fingerprintsensor as claimed in claim 4, wherein the number of the differentialamplifiers is equal to the number of the data electrodes.
 6. Thefingerprint sensor as claimed in claim 1, wherein the common electrodeis a specific data electrode in a specific column of the pixel array,and the signal of the specific pixel is obtained by looking at thesignals of the pixels surrounding the specific pixel and interpolating avalue in each row of the specific column.
 7. The fingerprint sensor asclaimed in claim 1, wherein the first substrate and the cover layer areintegrally formed.
 8. The fingerprint sensor as claimed in claim 1,wherein the first substrate is relatively flexible.
 9. The fingerprintsensor as claimed in claim 1, wherein the switching devices included inthe pixel array are thin-film-transistors (TFTs).
 10. A fingerprintsensor, comprising: a first substrate having a plurality of pixelsformed therein, the pixels being arranged in a pixel array of R rows andN columns, each pixel comprising a switching device and a capacitiveplate; a plurality of R row addressing electrodes, each such rowaddressing electrode being coupled to the switching devices of thepixels in a corresponding row of the pixel array for selectivelyenabling the switching devices in the corresponding pixel row; aplurality of N data electrodes, each such data electrode being coupledto the switching devices of the pixels in a corresponding column of thepixel array for sensing a signal provided by the capacitive plate of thepixel located at an intersection of a selected pixel row and thecorresponding column of the pixel array, wherein a selected one of theplurality of data electrodes arranged between two adjacent dataelectrodes serves as a reference electrode; a cover layer overlying thefirst substrate for receiving a fingertip of a user; and a plurality ofdifferential amplifiers, each differential amplifier having a firstinput coupled to a corresponding one of the plurality of data electrodesand having a second input coupled to the reference electrode, eachdifferential amplifier having an output terminal for providing an outputsignal, the output signal provided by each differential amplifier beingrepresentative of the difference between the data signal provided by itscorresponding data electrode and a signal provided by the referenceelectrode.
 11. The fingerprint sensor as claimed in claim 10, furthercomprising: a second substrate being different from the first substrate,the second substrate comprising a semiconductive material; and anintegrated circuit formed within the second substrate, wherein theplurality of differential amplifiers being formed within the integratedcircuit.
 12. The fingerprint sensor as claimed in claim 10, wherein thefirst input of the differential amplifier is a non-inverting input, andthe second input of the differential amplifier is an inverting input.13. The fingerprint sensor as claimed in claim 10, wherein the number ofthe differential amplifiers is equal to the number of data electrodesminus one.
 14. The fingerprint sensor as claimed in claim 10, whereinthe common electrode is a specific data electrode in a specific columnof the pixel array, and the signal of the specific pixel is obtained bylooking at the signals of the pixels surrounding the specific pixel andinterpolating a value in each row of the specific column.
 15. Thefingerprint sensor as claimed in claim 10, wherein the first substrateand the cover layer are integrally formed.
 16. The fingerprint sensor asclaimed in claim 10, wherein the first substrate is relatively flexible.17. The fingerprint sensor as claimed in claim 10, wherein the switchingdevices included in the pixel array are thin-film-transistors (TFTs).